Along with the development of broadband networks and high-performance information terminals such as mobile telephones and PCs, services of delivering content such as large volume videos and music over networks are increasing.
In a video delivery service, when videos of SD (Standard Definition) quality are transmitted, a large quantity of data communications of about 6 Mbps (data quantity of 6 megabits per second) are required for MPEG-2 (Motion Picture Expert Group-2). If any data loss or communication delay occurs during communications, videos may be distorted or video reproduction may be stopped due to delayed arrival of videos, which degrades the user's QoE. As such, a conventional video delivery service has been performed in such a manner that a communication provider secures a large-capacity band in order to ensure QoS (Quality of Service) so as to prevent data loss from arising on the way.
However, along with the widespread use of broadband Internet in recent years, video delivery services over networks, in which QoS is not assured such as the Internet, are increasing. The Internet has characteristics that an available network band varies depending on communications of other users. Further, higher-speed mobile networks brought about by LTE (Long Term Evolution) or the like and higher-performance terminals resulting from widespread use of smartphones are outstanding. As such, needs for viewing videos using mobile networks are increasing. Mobile networks have characteristics that a network band varies widely depending on, in addition to communications of other users, variations in the radio field intensity between a base station and a user terminal caused by the movement of the user, and effects of other wireless devices.
A delivery service for transmitting media data (data representing content) over such networks, in which QoS is not assured, is disclosed in Patent Document 1.
In the case of transmitting media data over a network in which QoS is not assured, when the band which is available in the network (available band) is narrowed, the entire media data cannot be transmitted. As such, videos may be distorted due to a packet loss, or reproduction of media data may be stopped due to delayed arrival of packets. Distortion of media data and stop of reproduction of media data cause user dissatisfaction, whereby the user's QoE is degraded.
In order to cope with such a problem, Patent Document 1 discloses a technology which uses a transmission rate and a media rate, when transmitting media data from a server device to a client device, as variables, and calculates a combination of a transmission rate and a media rate with which the QoE becomes the highest, based on an objective function associating those variables with QoE, and predetermined constraints. More specifically, among combinations of media rates and transmission rates satisfying the constraints, a combination of a media rate and a transmission rate with which the value of an objective function becomes the smallest is calculated. Then, media data of the calculated media rate is transmitted from the server device to the client device at the calculated transmission rate. Here, a transmission rate is a quantity of media data which is transmitted per unit time, and a media rate is a quantity of media data which is required for reproducing content for a unit time at a constant speed.
In the above-described objective function, the value thereof becomes smaller as the transmission rate is lower or the media rate is higher. Further, the above-described constraints include a limitation that a transmission rate does not exceed an available band and a condition that a media rate is any one of predetermined media rates.
By setting an objective function such that the value thereof becomes smaller as the transmission rate is lower, a lower transmission rate is selected preferentially, and a probability that congestion is caused in the network can be lowered, so that occurrence of a packet loss can be reduced. Further, by setting an objective function such that the value thereof becomes smaller as the media rate is higher, a higher media rate is selected preferentially, so that the content quality can be improved. As such, the technique described in Patent Document 1 is able to improve QoE.
Patent Document 1: WO 2011/018868